1. Field of the Invention
The invention pertains to the field of dehumidifying dryers. More particularly, the invention concerns a process and an apparatus to control the rotation speed of electric motors which are connected to turning devices used in dehumidifying dryers.
2. Description of Related Art
Electric motors are used to drive turning devices, especially blowers and rotors, containing the desiccant, as they are used in adsorption dryers as described in the present inventor's U.S. Pat. Nos. 6,951,065 and 5,688,305, which are incorporated herein by reference.
FIG. 1 (which is FIG. 2 in U.S. Pat. No. 6,951,065) shows such a dryer (2), representing an adsorption or refrigeration dryer. The granulate hopper (1) has a conveyer unit (4) at the top which sucks granulate out of granulate container (5) and lets it drop into granulate hopper (1) whenever level sensor (6) demands it. Dried granulate leaves the granulate hopper via duct (7) to a fabricating machine (3).
Return air leaves the hopper (1) via duct (8), sucked by blower (9) through filter (10), cooler (11) and dryer (2). Dry air from the dryer (2) flows via duct (12) through heater (13), where it is typically heated to 60-200° C., and is introduced into the lower part of the drying hopper (1). The dry air now flows from the lower part of the hopper (1) to the upper part of the hopper through the slowly descending granulate. In this passage, the air heats and dries the granulate. Finally, the air leaves the hopper as relatively cool return air through duct (8).
A return air temperature sensor (14) for measuring the temperature of the return air is located in the return air duct (8) just behind the hopper (1). A granulate temperature sensor (15) for measuring the granulate temperature is located inside the conveying unit (4), or, alternatively may be located (15a) in the granulate container (5). Both sensors are connected through the lines (21) (or (21a)) and (22) to control device (23), which alternatively controls flapper valve (24), which is located in duct (8), or the rotation speed of dry air blower (9). One or more air flow sensors (35, 36, 37), for example sensor (37) in duct (12) may be provided as might be needed by a given application. It will be understood that these sensors are shown for illustration purposes, and a given dryer controller might include inputs for all of the named sensors, or only one or two of the sensors, or for additional sensors not specifically called out here.
A second blower (90) can be arranged in line (8) before the return air stream enters the dryer (2). Blower (9) is controlled by controller (23) to control the volume of air flow for heating and drying in accordance with the invention, while blower (90) is controlled by controller (23) to regulate the air pressure in the line between blowers (90) and (9).
FIG. 3, which is FIG. 3 from U.S. Pat. No. 6,951,065, shows an adsorption dryer embodiment which uses a honeycomb rotor containing the adsorption medium, a system similar to the one shown in U.S. Pat. No. 5,688,305. The elements which are common with the embodiment of FIG. 1 have the same reference numbers, and will not be separately discussed here.
Blower (9) sucks return air through duct (8) and blows it through duct (25), and through section (26) of slowly rotating rotor (27), where the return air is being dried.
A regeneration blower (28) blows room air through heater (29) and through segment (30) segment (38) of the rotor (27) where the regeneration takes place. The moist regeneration air leaves segment (38) through the duct (39).
In order to improve the energy efficiency of the dryers under various load conditions in an economical way and with a minimum of a temperature increase in the air streams the rotation speed of the motors are changed through frequent on/off connections to the electric power line. In the past, this has been determined through a complicated calculation algorithm as described in the German Utility Patent (Gebrauchsmusterschrift) DE 201 03 438 U1. Such methods are costly in their development and their purchase costs.
Dryers of a given size must be able to dry different amounts of granulate per time. In real life applications the granulate through-put per hour varies as well as its moisture content. In order to solve this task in an economical way with a minimum use of energy, it is possible in accordance with the aforementioned patents to adjust the amount of dry air, of the regeneration air and of the rotation speed of the rotor, containing the desiccant to the drying test to be performed.
These volumes of air can be adjusted by flapper valves. The reduction of the air volume through a flapper valve has a disadvantage that it does not reduce the energy consumption of the electric motor at the same time. In addition it creates a temperature increase in the air stream with a result of lowering the efficiency of the adsorption dryer.
Another possibility to control the air volume is possible by changing the rotation speed of the electric motor through the use of frequency converters which can change the frequency or voltage of the electric power connected to the motor. One disadvantage is the high cost of a frequency converter. But even more important, if a frequency converter reduces the rotation speed of a blower the overall efficiency of the electric motor, the frequency changer and of the blower is being reduced resulting in an increase of the temperature of the air flow being reduced. This has a highly negative effect on the ability of the desiccant to adsorb moisture.